Measurement test instruments, such as oscilloscopes, logic analyzers, optical time domain reflectometers, and the like, are increasingly using microprocessor and raster scan display technology to provide users with greater measurement capabilities and ease of use. One feature that is useful in measurement test instruments is the ability to expand a portion of a displayed waveform to see details that are not easily discernable in the waveform. Waveform expansion is accomplished in current optical time domain reflectometers by incrementing the horizontal and vertical scales on the display by set amounts. For example, the OF235 Optical Time Domain Reflectometer, manufactured and sold by Tektronix, Inc., Beaverton, Oreg., has a horizontal scale in distance/division that ranges from 5000 meters to 5 meters per division in a 5-2-1 sequence. The vertical scale is in dB/division with scale setting of 5 dB, 1 dB and 0.25 dB per division. Decreasing the distance/division and/or the dB/division setting has the effect of expanding the waveform in the display. In operation, a movable distance cursor is positioned on the waveform at a location where an operator wishes to expand the waveform. Changing the distance/division setting automatically repositions the distance cursor and its associated waveform point as close as possible to the center of the screen while effectively expanding the waveform as a function of the distance/division setting.
Expanding portions of a display have also been successfully used in the graphic display terminals and computer workstations where images, such as maps, mechanical parts, integrated circuit and circuit board layouts, and the like, are displayed on a cathode ray tube or the like. U.S. Pat. No. 4,532,605 entitled "True Zoom of a Displayed Image" describes an apparatus and method for use with a graphic display terminal where a particular region of a displayed image in the display is defined and expanded while maintaining the width of the lines of the zoomed image regardless of the degree or amount of magnification of the zoomed image. The zoom function is initiated by a user hitting a zoom key on the terminal keyboard and rotating thumb wheels to vary the vertical and horizontal size of a box defining the zoom region. The initial size of the box is equal to the full screen display and its center point is set to the center of the display. Once the size of the zoom region is defined in the horizontal and vertical dimensions, the user hits a pan key which displays cross-hair cursor at the center of the defined zoom region and allows movement of the region over the display. After the region is positioned at the desired location, the user hits a view key, which expands the zoom region to fill the screen.
U.S. Pat. No. 4,812,996 entitled "Signal Instrumentation Control System" describes the use of a computer to control a programmable test instrument, such as a digitizer, for displaying data acquired from the test instrument on the computer display. The system is interactive in that once the data is acquired from the test instrument and displayed, the user may define a smaller region on the display for further examination. The computer keyboard or mouse is used to move a cross-hair cursor over the display for defining diagonal corners of the region that is to be re-examined. Software in the computer translates the defined corners of the region from the display coordinates of the computer to test instrument parameters, such a sample interval, delay, and vertical range, and transmits these parameters to re-program the instrument. The test instrument acquires the new data at the new parameters setting and passes the data back to the computer, which translates the data into screen coordinates and displays the newly acquired data with the appropriate scale settings.
Each of the above described systems for expanding waveform or image data on a display has drawbacks. Expanding a waveform as a function of predefined horizontal division settings limits the amount of waveform expansion to the defined settings. In addition, expansion of the waveform is sequential requiring an operator to take the time to decrement through the distance/division settings in order to expand the waveform. Graphic display terminal pan and zoom expansion has a similar drawback to the incremental horizontal distance settings in that only one expansion can be performed using the defined expansion window. A new window must be defined and positioned on the display in order to perform additional expansion. Similarly, the computer controlled programmable test instrument requires the redefining of the expansion window and the reacquisition of data for each new window.
What is needed is a system for expanding a waveform in a display area of a measurement test instrument, such as an optical time domain reflectometer, where an expansion window is definable relative to the waveform in the display area and where that portion of the waveform being defined by the expansion window is displayable in an expanded form in the display area with the expanded waveform being further expandable in the display area in a continuous manner.